Dispersing gas into molten metal

Abstract

Gas is dispersed into molten metal by immersing a shaft-supported impeller into the molten metal and by pumping gas through the shaft and out of the impeller. The impeller is in the form of a rectangular prism having sharp-edged corners that shear the gas into finely divided bubbles. The particular shape of the impeller provides an especially effective bubble-shearing and bubble-dispersing action.

What is claimed is: 
     
       1. A method for dispersing gas into molten metal, comprising the steps of: providing an impeller in the form of a rectangular prism having upper and lower faces, a width (A), a depth (B), and a height (C), with (A) being equal to (B), the impeller having a gas discharge outlet opening through the lower face of the prism;   providing an elongate, rotatable a shaft rigidly connected to the impeller and projecting from the upper face of the impeller;   providing means for conveying gas to the gas discharge outlet;   providing a vessel within which molten metal is contained;   immersing the impeller into the molten metal contained within the vessel;   rotating the shaft about its longitudinal axis; and   pumping gas through the gas discharge outlet while rotating the shaft.   
     
     
       2. The method of claim 1, wherein the gas discharge outlet is defined by an opening extending through the upper and lower faces of the impeller, and the means for conveying gas to the gas discharge outlet is a longitudinally extending bore formed in the shaft, the shaft being connected to the impeller such that the bore in the shaft and the opening in the impeller are in fluid communication with each other. 
     
     
       3. The method of claim 2, wherein the outer surface of the shaft and the inner of the opening in the impeller are threaded, and the shaft is connected to the impeller by threading the shaft into the opening. 
     
     
       4. The method of claim 1, wherein the shaft is connected to the impeller by means of a threaded connection. 
     
     
       5. The method of claim 1, wherein the shaft is connected to the impeller at the center of the upper face. 
     
     
       6. The method of claim 1, wherein the shaft is cylindrical. 
     
     
       7. The method of claim 1, wherein the impeller and the shaft are made of graphite. 
     
     
       8. The method of claim 1, wherein C equals 1/3 A. 
     
     
       9. The method of claim 1, wherein molten metal is contained within a vessel having an inner diameter (D), the impeller is centered within the vessel, and the ratio of A to D is within the range of 1:6 to 1:8. 
     
     
       10. The method of claim 1, wherein the shaft is rotated within the range of 200-400 revolutions per minutes.